Water (H2O) is a strong and aggressive solvent that interacts with and dissolves most substances it meets. Contaminants include atmospheric gases (oxygen, nitrogen, and carbon dioxide), dissolved minerals and organic substances, and suspended colloidal matter. Water also provides an ideal environment for the growth of bacteria and other microorganisms if the necessary nutrients and conditions for growth exist. As a result, all water in nature contains impurities, such as minerals, salts, various metals, and other compounds. In fact, drinking water can be a beneficial source of various compounds. However, in other uses, impure water is an issue. The purity of water can be tested by measuring the resistivity/conductivity of the water.
Electrical conductivity is a measure of a material's ability to conduct an electric current. Resistivity, the reciprocal of conductivity, is the measure of how strongly a material opposes the flow of electric current. Accordingly, a resistivity value can be determined from a conductivity value, and vice versa.
An electrical current results from the motion of electrically charged particles in response to forces that act on them from an applied electric field. Ultra pure water is a low conductor of electricity because it has a small number of electrically charged particles that can flow in response to an electric field. It has a very low conductivity of 5.5×10−6 S/m. When water is impure, it has a significant number of charged particles, free ions, dissolved in solution. These particles move in response to an applied electric field, and as a result, this makes impure water a good conductor of electricity. Therefore, a measure of the purity of water can be obtained through the measurement of its conductivity/resistivity values. The higher its conductivity and the lower its resistivity, the more impure the water sample is.
Depending on the type and concentration of contaminants, most natural waters are not suitable for potable use much less for most research and industrial applications. Most all municipalities and other purveyors of potable water provide some level of water treatment to make the water suitable for consumption.
Potable water is too contaminated for many applications. Instead, even more highly purified water is required. For example, high-purity water and/or ultra-pure water are typically produced and used in, but not limited to, the following industries: microelectronics manufacturing, semiconductor manufacturing, pharmaceutical manufacturing, photovoltaic manufacturing, power generation Industry, nuclear industry, chemical laboratories, and hospitals.
In selected applications, the purity requirements are very high. Water used in manufacturing electronic components, must be substantially free of extraneous minerals, particles, organisms, organics and dissolved gases. Water purity requirements specified by some semiconductor manufacturers have reached particularly demanding standards. For example, in terms of mass, required purity standards correlate to contaminant levels at or below microgram (μg) levels per liter of water. Exemplary requirements can be found from ASTM International, which provides listing of standards for water purity, among other things, including ASTM D5127-07, entitled “Standard Guide for Ultra-Pure Water Used in the Electronics and Semiconductor Industries,” which is herein incorporated by reference, for all purposes.
To purify water to such exacting standards, various treatment processes can be used synergistically, to include filtration, membrane separation, ion exchange, degasification, UV sterilization, and UV oxidation of dissolved organics, to name a few. In each of these processes, measurements are taken of the water stream as it progresses through the process. As a result, one can ensure that the system is functioning properly.
Water must be constantly monitored during the purification process to check the purity of the water at different stages of the process and ensure that the purification process is working. One chemical process that is employed to purify water is the ion-exchange process. In this process, water is filtered through ion-exchange resin or ion-exchange polymer beads, which contain certain ions on their surface. As the water flows over the beads, the impurities in the water are picked up by the beads, which then release the ions on their surface into the water. High purity water can be produced using ion-exchange processes or combinations of membrane and ion-exchange methods. Cations are replaced with hydrogen ions using cation-exchange resins; anions are replaced with hydroxyls using anion-exchange resins. The hydrogen ions and hydroxyls recombine producing water molecules. Thus, no ions remain in the produced water. The purification process is usually performed in several steps with “mixed bed ion-exchange columns” at the end of the technological chain.
As the hydrogen and hydroxyl ions on the resins are displaced by the ions in the water being filtrated, the process reaches equilibrium with a much lower ion concentration in the water than was started with. At this point, no more ions are filtered out of the water by the resins, and the ions “breakthrough” the ion exchange resin.
Because of this “breakthrough” effect, the conductivity/resistivity of the water must be continuously monitored to ensure the purity of the filtered water. If equilibrium is reached and an unmitigated “breakthrough” effect occurs, the water is no longer pure and contamination of the downstream processes occurs.
There are several methods and instruments for examining resistivity/conductivity of water, but these contain several limitations. Technical problems and limitations still exist in resistivity/conductivity instrumentation. For example, such sensors produce measurements that vary slightly from one to the other, even when measuring the same solution at the same time, even with the same sensor model from the same manufacturer. Such variations can result from minor variations in construction, materials, and so on. To account for such variations and improve accuracy, the sensors are calibrated in the factory, where an offset value or cell constant is determined for the sensor.
In measuring water having high levels of purity, sensor error becomes a significant, limiting factor on accuracy. Currently, typical resistivity sensors have an upper limit in accuracy around one to two percent (1%-2%) over the instrument range. As a result, existing resistivity/conductivity instruments can and often do output values that are beyond theoretically capable values. Further, current sensor systems are unable to calibrate conductivity instrumentation in the field; existing instruments are typically factory calibrated.
However, after repeated use, the instrumentation will lose its calibration and accuracy, and field calibration is necessary and more cost-effective than the purchase of new instrumentation. Sometimes such instruments would provide output readings that exceed “theoretical” values of resistivity for theoretically pure water (e.g., ≈ greater than 18.1 mega-ohm (mΩ) at 25 degrees Celsius). Such measurement inaccuracies can limit the effectiveness of purification processes as well as increase associated costs.
It should be appreciated that there remains a need for a system and related method improved sensor systems for accurately measuring resistivity of highly pure water. The present invention fulfills this need and others.